Analyzing of Modified DES-64 Algorithm for Securing the Data on File Storage Systems

. Rapidly developing technology makes it easier for us to get information from processed data. The authenticity of a data is a very basic thing of information. With the rapid development of technology, there are more and more threats such as manipulate, phishing or even deleting data that will be processed into information without authentication. Guaranteeing the confidentiality and authenticity/integrity of data in the form of files is very important to produce accurate information and can be accounted for by way of encryption or one way function for authentication. The goal is of course to make it difficult for other parties to change it. The ultimate goal of this study is to implement the modified des 64 algorithms, to compare the avalanche effects and randomness test values from the standard DES test results with modified DES on the S-Box and P-Box sections which are made dynamic depending on the key, so that the data stored is encrypted. The modification process is by entering several key combinations including the DES weak key, alphabet and alphanumeric. The implementation is carried out on document file storage with a user authentication process and file encryption with a key that uses sha-256 to generate a different key for each file. When the owner downloads the data, the decryption process is executed, so that the security and authenticity of the data can be guaranteed. With the modified 64-bit DES, it is expected that data security problems on storage can be decreased or even solved.


Introduction
Rapid technological developments make it easier for us to obtain information that has been processed from data, the fundamental thing of information is the authenticity of the data.Information security is a step so that we can prevent or detect fraud in an information-based system.Interception (wiretapping to retrieve confidential data without authentication), modification and fabrication (adding data without authentication) as in the new case related to the Dukcapil data leak which resulted in the widespread dissemination of millions of important Indonesian data [1].Maintaining confidentiality and data integrity of data is very important to produce correct information, both public information, especially information that can be accessed with limited access rights.A collection of various related information that is also stored in secondary storage is a file.Conceptually, files have several types, some of which are data types consisting of numeric, character and binary.Many things are done to secure a file, including by limiting user access rights, maintaining the confidentiality and integrity of the data, namely by encrypting and decrypting it which will make it difficult for other parties to change it either during the storage process or after storage.To strengthen data authenticity and integrity, user data authentication functions are used, one of which is by using a one-way function/hash.
In 2022, research was carried out regarding the implementation of standard DES on student scores with key input in the form of passwords created by users [2], research in 2021 concluded that implementing standard DES for data security makes our data safe from attacks [3].In 2019 it was argued that the weakness of the DES standard lies in its weak keys [4] The goal of this research is to find out and compare the avalanche effects and randomness test values from the results of the standard DES test with the DES modification of the S-Box and P-Box sections, as well as develop an encryption-decryption algorithm that can later be used by other online and offline-based systems storage.The process carried out is to authenticate the user and encrypt the uploaded data with a different key for each file and can only be decrypted by users who have authentication.By modifying the 64-bit DES algorithm, it is expected that the encryption process time will be better, the attacker will find it more difficult to decrypt the uploaded file.

DES Algorithm
Data Encryption Standard is a symmetric algorithm published since 1977 where encryption is carried out on 1 block of plaintext data of 64 bits and a 56-bit key.Plaintext must go through 16 stages of encryption (iterations), Fig. 1. is a schematic of the DES algorithm.The process of encrypting the plaintext block is carried out after the initial permutation and each plaintext block undergoes 16 rounds of encryption.Each round of encryption is a Feistel network.Fig. 2. is a fiestel computing diagram where E is an expansion function that expands R i-1 block which is 32 bits long into a 48-bit block.Expansion function with expansion permutation matrix in Table 1.The result of the expansion is E(R i-1 ), which is 48 bits long, xored with Ki which is 48 bits long to produce vector A which is 48 bits long.

𝐸 𝑅 𝑖−1 ( ) ⊕ 𝐾 𝑖 = 𝐴
Vector A is grouped into 8 groups, each with 6 bits, and becomes input for the substitution process.The substitution process is carried out using eight S-Boxes which receive 6-bit input and produce 4-bit output per S-Box.The first 6-bit group uses the first S-Box and so on.The eight S-Boxes are in Table 2. Output of the substitution process is a vector B which is 48 bits long.Vector B becomes input for the permutation process to randomize the results of the Substitution-Box (S-Box) process.Permutations are carried out using the Permutation-Box (P-Box) matrix in Table 3. Bits of P(B) is output of f function.Bits of P(B) xored with L i-1 for getting R i Fig. 3.
Output of round i (

Fig. 3. Scheme for finding Ri
The most important part of the DES algorithm is the S-Box which is a function to map a 6-bit binary value to a 4-bit binary.DES has 8 S-Boxes which are non-linear functions (the output value cannot be used to find the input value of the S-Box).The weakness of the DES algorithm is that the effective key size is only 56 bits, which makes it vulnerable to brute force attacks [5].

DES 64-BIT S-Box Modification
Modification of 64-bit DES is to increase the complexity and increase the immunity of the DES-64-bit algorithm.The modifications made to the S-Box is intended to increase the confusion and diffusion of 64-bit DES.
The generated S-Box value is xor-ed with the results of the key xor so that the S-Box

DES 64-BIT P-Box Modification
Modification of 64-bit DES is to increase the complexity and increase the immunity of the DES-64-bit algorithm.The modifications made to the P-Box is intended to increase the confusion and diffusion of 64-bit DES.

Results and discussion
The results and discussion of tests that have been carried out on the modified 64 bit DES algorithm by calculating the avalanche effect value and randomness test.

Avalanche Effect Test
One of the characteristics to determine whether a cryptographic algorithm is good or not is to look at the avalanche effect.Small changes to the plaintext or key will cause significant changes to the resulting ciphertext [6].In other words, changing one bit in the plaintext or key will result in changing many bits in the ciphertext.An avalanche effect is said to be good if the resulting bit changes range between 45-60% (50% is a very good result).This is because these changes mean it makes a difference that is quite difficult for cryptanalysts to carry out attacks.
Avalanche effect is an important characteristic for encryption algorithms by changing one bit in the plaintext and it can be seen by changing one bit in the plaintext and look at the result of changing in a half of the bits in the ciphertext at least.The purpose of the avalanche effect is to change only one bit and there is a big change in the ciphertext, so it is difficult to do ciphertext analysis when it is attacked [7].The results of the Randomness Test carried out with Cryptool 1.4.3 software show that the modified DES-64 bit algorithm has passed the five basic tests for randomness.Table 4 is a result of avalanche effect calculation with the encrypted plaintext is "ilkom2012" and key "00".

Randomness Test
Randomness test is a series test to see randomness.The purpose of the series test is to determine whether there is a certain pattern in the data or whether the data is a random sample.From carrying out this randomness test, 2 possibilities will be obtained, namely if the data is random towards the median then the data is homogeneous, whereas if the data has a trend that is more above the median or below the median then the data is not homogeneous.

Conclusions
The conclusion of this research is that the modified algorithm increases the value of the avalanche effects and randomness test and the value of the avalanche effects and randomness test DES with the S-Box modification increases significantly compared to DES with the P-Box modification and DES S-Box P-Box so that it increases the complexity 64 bit DES algorithm.The results of the limited test with entering files show that the DES 64-bit cryptographic algorithm with modifications to the S-Box P-Box depends on entering messages, if the message entered is large, the processing time for encryption will be longer.

Fig. 4 .
Fig. 4. S-Box Modification of DES Mod 16 because in the DES S-Box the maximum value of 1 element is 16 so if you don't mod the S-Box element value can be > 16.After finding the xor Key, then xor each element in the S-Box with the xor key.This process will continue to repeat so that the lines from S-Box[0] to S-Box[8] will have a different key from the S-Box in general and also make the S-Box elements more dynamic.

Table 3 .
[5]mutation Box[5] value for each generation will be different and depend on the key, the process of creating a S-Box that depends on the key starts by performing a xor operation on each key element as

Table 4 .
Result of avalanche effect test

Table 5 .
Randomness Test Encrypted File 351 KB

Table 6 .
Size of Encrypted and Decrypted File

Table 7 .
Size of Encrypted and Decrypted File